1. Field of the Invention
The invention relates to an electron-optical corrector for eliminating third-order aberrations, such as spherical aberrations, field curvature and off-axis astigmatism. The corrector being devoid of third-order off-axis coma, third-order distortion and first-order chromatic aberrations of the first degree. The corrector has a construction which is symmetrical about the central plane in the direction of the linear optical axis. A hexapole S1 of length l1 is first positioned in the direction of the beam path, followed by a circular lens R1, a hexapole S2 of length l2.
2. Description of the Background Art
The efficiency of electron-optical systems, which in the sense of this invention are also understood to include those with ion-imaging systems, is limited by their image aberrations, of which, depending on the specific application and the extent of the corrections already made, particular image aberrations are responsible for limiting the performance, the elimination of which represents considerable progress in the improvement of electron-optical systems. It is possible to systematically subdivide and classify the image aberrations into axial image aberrations, which are also determined by the fundamental paths emerging in the two sections of the optical axis in the object plane, off-axis image aberrations, which in turn are dependent on the fundamental paths emerging outside the optical axis in the object plane, and chromatic aberrations, which only occur with different speeds of the imaging electrons. With magnifying electron-optical systems, such as those used in electron microscopy, it is most important to eliminate the axial image aberrations to increase efficiency. With reducing electron-optical systems, such as those used in lithography for inscribing objects by means of electron beams, the elimination of off-axis image aberrations is decisive.
The aim is always to set up and adjust, in its entirety, the system comprising the imaging lens system and the corrector such that the efficiency-limiting image aberrations of the entire system are eliminated or substantially minimised, the corrector having the function of, on one hand, achieving, by negative image aberration coefficients, an elimination or at least a reduction, and on the other hand not causing an increase, of disadvantageous image aberration coefficients.
Optik 69 No. 1 (1984) 24-29 discloses the use of two hexapoles for the correction of electron-optical image aberrations in circular lenses. These correction elements are also used for eliminating the first order spherical aberration (Optik 60 No. 3 (1982) 271-281).